The major objective of this project is the elucidation of the mechanism and regulation of the interaction of mRNA with the ribosomes. Cryptobiotic gastrulae of Artemia salina contain a store of inactive mRNP. The major protein component of these mRNP is an RNA helix-destabilizing protein termed HD40 (MW 40,000); it was purified to homogeneity. The protein unfolds the secondary structure of polynucleotides with a stoichiometry of one protein per 12-15 nucleotides. Addition of HD40 in excess of one molecule per 12-15 nucleotides results in the formation of distinct bead-like structures along the nucleic acid strand. The beads are 20-35 nm in diameter with a center to center distance of about 40 nm. The structures formed in vitro by HD40 are extremely similar to the known "beads-on-a-string" structures of hnRNP isolated from many eukaryotic cells. The amino acid composition of HD40 is very similar to that of a class of nuclear RNA binding proteins. Antibodies to HD40 cross-react with nuclear proteins. Bead formation is an unusual feature of HD40; it has not been observed with any known HD protein. Present work is focused on the disappearance of the inactive form of mRNP during the development of the cysts while the amount of the actively translated polysomal mRNP increases. The events in protein synthesis upon resumption of development of dormant A. salina embryos are strikingly similar to those in newly fertilized eggs. We intend to investigate the fate and the function of protein HD40 in development and the relationship between HD40, a cytoplasmic protein, and nuclear RNA binding proteins. The unusual "beads-on-a-string" structures of the HD40-polynucleotide complex will be investigated quantitatively.